Quick and dirty WAI test
 

I pulled off the top cover over the air filter so as to pull intake air from the engine compartment instead of the duct to the wheel well.

At 90 degrees outside, the intake air temperature increased from 98 to 100 deg F up to 120-130 deg F. The MPG seemed to drop a little, so I looked at the timing. At 8 PSI MAP and 1800 RPM (55 MPH), the timing was 30 degrees. Put the cover back on, the intake air temperature dropped back to 98-100 degrees, and the timing changed to 35 degrees at the same MAP and RPM.

Seems to me that increasing IAT causes the knock sensor to pull timing, leading to a net reduction in MPG.

I had ideas of building a WAI with a vacuum operated flapper to automatically switch to CAI at less than about 5 In Hg vacuum, but I'm dropping that and plan to work on a better air dam instead.

I moved the IAT sensor on my Magnum from right before the throttle body (and after my WAI heater) to right before my air filter (and before my WAI heater). That took care of my ignition timing retard issue.

My truck doesn't seem to have this issue. I got 22 MPG recently, carting a car door 35 miles. That's without the aerocap in place.